Depositional and diagenetic drivers of a heterogeneous reservoir quality in the Neogene strata of the Red Sea (Egypt)

doi:10.21203/rs.3.rs-5295690/v1

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Depositional and diagenetic drivers of a heterogeneous reservoir quality in the Neogene strata of the Red Sea (Egypt)

https://doi.org/10.21203/rs.3.rs-5295690/v1

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Petrophysical analyses is the most important, but sophisticated, method for assessing reservoir potential. Herein, we analyzed the Neogene sediments that extend along the Red Sea coast between Wadi Khasheir to Ras Honkorab on South Egypt to evaluate their hydrocarbon potential and to characterize factors behind their evolution. A total of 311 rock samples were collected from 32 measured sections. One hundred and fifty thin sections were made for microfacies analyses. The petrophysical measurements were performed on 185 plug samples representing the different formations. A high porosity was recorded within the conglomerate and greywacke petrofacies of the Miocene Ranga Formation, with an average of 14.5% and have permeability up to 3806 and 3516 mD, respectively. Furthermore, the boundstone microfacies of the Miocene Um Mahara Formation have the highest porosity values with an average of 22.2% and permeability up to 1190 mD. The diagenetic pathways were influenced by the depositional characteristics such as composition in greywacke and feldspathic arenite petrofacies, texture in boundstone microfacies, and the primary porosity as well. The presence of felspars and interparticle pores enhanced the secondary porosity and pore throat size via dissolution. The presence of bitumen as pores-filling material in the Pliocene-Pleistocene sediments reveals their importance as hydrocarbon reservoirs in the subsurface. This tectonic configuration of different sub-basins have controlled the sedimentation and the digenesis processes.

Petrophysics

pore throat

dissolution

Neogene

Red Sea

Egypt

Studying the petrophysics of the outcropped rocks is important in predicting their reservoir characteristics and quality in case they are buried and have the potential to store and transmit fluids (Ali et al., 2018; 2022a). The response of the petrophysical parameters to the depositional features and the changes caused by the diagenetic processes is vital in understanding the rock behavior under different environmental circumstances (Ali et al., 2022b; 2023; 2024). The diagenetic processes affecting rocks can erase the rocks’ original depositional characteristics (Johnsson 1993; Worden et al., 2018).

The Red Sea rift basin was formed during the Oligocene/Miocene periods about 27–23 Myr (McClay et al., 1998; Bosworth & McClay, 2001). It is believed to have originated due to the break-up and separation of the African Craton from the Arabian block (Fairhead & Green 1989). The early initiation of subsidence and uplifting of the rift shoulder sediments might have started in the Eocene/Oligocene periods. The second phase of rifting occurred within the Middle Miocene, and it may be currently active (Tammam, 2003). So, the Red Sea represents one of the most currently active sedimentary basins that is still developing.

The surface sedimentary sequence of the Red Sea coastal strip has an average width of seven kilometers. The Miocene was a period of great transformation, leading to the present Red Sea coast sediments and similar sedimentation in the northwestern part of the Gulf of Suez (Hewaidy et al., 2012; 2014). After the Oligocene uplift, an Early and Middle Miocene Tethys transgression began, followed by late Miocene regression due to the Alpine orogeny. So, more extensive marine beds were formed in the Miocene. Pliocene sediments are widely distributed along the Red Sea. The Quaternary was characterized by regression with the minor transgression of sea level. Uplift and tectonic disturbances mark the Pliocene-Quaternary boundary in the Red Sea region (Saleh et. al., 2006). Neogene sediments are widespread along the Egyptian Red Sea coast, especially in Marsa Alam and southwards. In this area, the Miocene, Pliocene, and Quaternary clastic sediments form good exposures, allowing detailed geological studies (Philobbos et al., 1989, 1993; El-Haddad et al., 1993).

From the economic point of view, Beydoun (1989) mentioned that the Arabian side of the Red Sea has the region's widest and most poorly explored shelf. The rift Neogene stratigraphy of the Red Sea is very similar to that of the prolific oil-producing Gulf of Suez Graben. Pre-Neogene rocks, however, are only present in patches. Neogene hydrocarbon traps of the Red Sea are provided by rotated Basement fault blocks and horsts in the pre-evaporite sequence. It is also represented by salt flowage, piercement, and stratigraphic pinch-out in the post-evaporite sequence. Petrophysical properties (porosity and permeability) of syn-rift sedimentary formations are varied and closely associated with facies types.

The present study represents the first detailed petrographical and petrophysical investigations of the Neogene sediments for the area between Wadi Khasheir to Ras Honkorab, Red Sea coast. These investigations would provide insights into the evolutions of the petrophysics of the Neogene siliciclastics and carbonate rocks in response to the original depositional features and the later diagenetic processes.

The study area extends for about 41 km in length and 10 km in width. It is delineated by latitudes 24⁰ 16^\, 24⁰ 34^\ N and longitudes 35⁰ 03^\, 35⁰ 21^\ E (Fig. 1). The area is characterized by the high mountain ranges of the Eastern Desert and the coastal sedimentary stretch along the Red Sea coast. It is dissected by many wadis that initiate the mountainous terrain and mostly run towards the Red Sea following the general eastwardly slope (Ahmed, 2001).

The investigated area is located between Ras Honkorab in the north and Wadi Khashir in the south and exhibits almost complete sections of Oligocene, Neogene sediments (Fig. 2). The sedimentary sequence includes Oligocene (Abu Ghusun Formation), Miocene (Ranga, Um Mahara, Siyatin, Abu Dabbab and Um Gheig formations), Upper Miocene?-Pliocene (Marsa Alam Formation), Pliocene (Shagra Formation) and Plio-Pleistocene (Samadai formation, Said, 1990; Purser & Boscence, 1998; Khalil & McClay, 2009). These Neogene sediments are arranged in three belts parallel to the Red Sea coast (El-Haddad, 1984; Philobbos et al., 1988). The inner belt (in the west) comprises the pre-evaporite Miocene siliciclastics and carbonates (Abu Ghusun, Ranga, and Um Mahara Formations). The middle belt consists of the main Abu Dabbab Evaporites, underlain by Siyatin Formation and overlain by Um Gheig Formation. The outer (eastern) belt comprises a siliciclastics-carbonate sequence (Marsa Alam, Shagra and Samadai formations).

Abu Ghusun Formation was known as the first syn-rift sediments along the Red Sea coast. This name has been introduced by Philobbos et al. (1988) to include all the fine red siliciclastic sediments. The maximum thickness of this formation was measured at the northern flank of Wadi Abu Ghusun which reaches up to 40 m. It non-conformably overlies the Precambrian basement rocks and unconformably underlies the greenish-grey conglomerates of Um Abas Member (Ranga Formation, Aquitanian-Burdigalian age (El Bassyony, 1982)), at the southern flank of Wadi Abu Ghusun. The Ranga Formation is capped by the reefal carbonate of the Late Burdigalian-Langhian Um Mahara Formation at the entrance of the northern flank of Wadi Abu Ghusun. It is represented by poorly-cemented red to brick-red conglomerates, conglomeratic sandstones, sandstones, and siltstones at the base and grey to yellowish grey and green sandstones at the top. Conglomerates are polymictic with lithoclasts of basement origin, moderately sorted, immature, and cemented by red sandstone, clayey, and calcareous materials. Occasionally, fine red clastics with abundant ferruginated materials are dissected by anhydrite veins.

Marine fossils are completely absent, but plant remains (roots) are recorded in the northern part of Wadi Abu Ghusun. At Wadi Khashir, Abu Ghusun Formation is characterized by silicified woods distributed within medium to coarse clastics. It overlies the basement rocks and unconformably underlies the Ranga Formation (Um Abas Member). Abu Ghusun Formation might be deposited as fluviatile (meandering) streams under arid to tropical humid oxidizing conditions (Mahran, 1986; Hasan, 1995).

A total of 311 rock samples were collected from 32 measured lithostratigraphic sections (covering all the megascopic litho-units) described along several wadis located on the Egyptian western coast of the Red Sea (Fig. 3). One hundred and fifty thin sections were prepared according to the change in the rock facies to investigate the mineralogy, petrography, and textures of these rocks to eliminate the depositional features and the diagenetic pathways. Microfacies, depositional texture, grain size, porosity type, and percentage, mineralogy, and diagenetic processes were described in the thin sections using polarized transmitted-light microscopy (Olympus BX51). No systematic grain size analysis was carried out, but the average grain size and sorting were estimated in the thin sections. The percentage of detrital, diagenetic constituents and pore types are expressed relating to the bulk volume of the rock. Some porous samples were impregnated with blue-dyed epoxy resin and then the thin sections were prepared. This was followed by impregnation under a vacuum and heat with blue-stained epoxy to augment sample cohesion and prevent material loss during the grinding procedure (some samples were ground in kerosene to minimize any potential for sample damage due to freshwater sensitivity). The standard facies types of Flügel (2010) and Wilson (1975) based on Dunham (1962) classification were used in the description of carbonate microfacies. The terms of Folk (1962 &1974), Pettijon et al. (1987), and Tucker (2013) were used to describe the sandstone petrofacies association. Scanning electron microscopy images were obtained from freshly broken samples coated with gold and examined by JEOL-JSM-IT200 at the Central Laboratory for Microanalysis, Minia University. Petroleum inclusions fluorescence analyses for about thirty thin sections of Miocene and Pliocene sediments were carried out at the Department of Genetics, Faculty of Agriculture, Minia University by using a Y-FL EPI-Fluorescence attachment, which was attached to an Olympus BX61 microscope coupled with a digital Olympus camera.

The petrophysical measurements were performed on one hundred and eighty-five plug samples representing the different formations with dimensions of 0.5 to 1-inch diameter and 1.5–3.5 cm length. The porosity of core samples is determined by mercury injection Porometer 1052 instrument designed to yield bulk volume and pore volume. Gas Permeameter1011-801 with a range of 0.5 to 3000 millidarcy (mD), is used to determine the permeability of core samples by flowing gases (air/ nitrogen) through the plugs.

4.1. Facies analysis

The facies types of the studied rock units were determined based on the lithologic content and the primary depositional features that were observed in the outcrops and the textures, faunal content, and sedimentary structures in the thin sections as well. The studied sequences include three main categories of facies: 1) siliciclastic; 2) carbonates, and 3) evaporite facies. The three facies were detected in the Miocene sequence while the Pliocene-Pleistocene sequence comprises only the siliciclastic and carbonates facies.

4.1.1. Siliciclastic facies

The siliciclastic facies is about 45% relative to the Miocene sediments and is recorded in both Ranga and Siyatin formations with the dominance of conglomerate (28.51%), sandstones (9.88%), and shales (6.59%).

The conglomerate facies of the Miocene age is polymictic in composition and its thickness ranges from 71m to 35m at Wadi Abu Ghusun and Ras Honkorab, respectively. The thickness diminishes toward the west where it is about 4m at Wadi Khashir. The color is dark grey to black, and the marine fossils are completely missing. The grains are subrounded to rounded, low sphericity, ill-sorted, open-packing, and reveal free or no contacts with a size range from 0.5 to 45cm (Fig. 4a & b & c). The cross-bedding is a common sedimentary structure within the relatively fine pebbly conglomeratic beds. The field observations indicate the presence of normal graded bedding from cobble to boulder conglomerates fining upward into coarse pebbles followed by very coarse sand at the uppermost part. The elongated pebble and cobble grains have an N80ºE direction which indicates the paleocurrent orientation. The mineralogy includes quartz (as mono- and polycrystalline), plagioclase feldspars, micas, and amphibole grains of igneous, sedimentary, and metamorphic origin. Another conglomerate facies of the Miocene sediments is a bioclastic conglomerate recorded in Um Diheisi Member at West Ras Honkorab and Siyatin Formation at Wadi Abu Ghusun with a thickness reaching up to 4m. The rock’s color is light yellowish to dark brown and consolidated. The bioclasts include coralline algal, foraminifera, and corals (Fig. 4d).

The polymictic conglomerate facies of the Pliocene-Pleistocene age have thicknesses from 2.5 to 6m, 1.5 to 43m, and 1.5 to 18m in Gabir, Abu Shiqeili, and Dashet El Dabaa members, respectively. In the Samadai Formation, it is represented by a coarsening upward sequence from coarse pebbles and cobbles at the base to boulders at the top (18m thick, (Fig. 4e). The mineral composition of these conglomerates is similar to those of the Miocene age besides the grains are embedded in sand and silt matrix with spar calcite cement (Fig. 4f). The bioclastic polymictic conglomerate of Abu Shiqeili Member ranges in thickness from 8m to about 20m and contains crystalline limestone intraclasts and micritized algal fragments (Fig. 4g). It is characterized by gravel grains with normal graded bedding and rich in black and brown hydrocarbon patches.

The sandstone facies of the Miocene age is mainly represented by arenitic sandstones with subordinate and local greywacke and arkosic sandstones. The arenitic sandstone petrofacies is represented by feldspathic lithic arenite and quartz arenite, where the former petrofacies is recorded in the upper part of Um Abas Member (Ranga Formation) at Wadi Abu Ghusun and in Siyatin Formation at Wadi Abu Ghusun and Ras Honkorab area with a thickness of 8m of semi-consolidated greyish white to yellowish grey sandstones which are laterally change into 1.5m thick of greyish black hard laminated sandstone. The mineralogy is made up of quartz grains as monocrystalline (41%) and polycrystalline (17.9%), plagioclase feldspar (17.9%), and basement rock fragments (23%) embedded in clay matrix (6.3%), calcite and evaporite cement. The rock framework exhibits coarse size, subangular, and poorly to moderately sorted grains with open packing at the base to close packing upwardly with dominant point contacts (Fig. 4h). The Quartz arenite facies is recorded in Um Abas Member as light yellow to brown, hard, bioturbated layer of 1m thickness. It is composed of monocrystalline quartz (about 90%), rock fragments (5%), feldspar (3%), and algal bioclasts supported by dolomitic cement (Fig. 4i). The grains are fine, angular to subangular, moderately to well sorted, and close-packed fabric which reveals straight contacts.

The greywacke sandstone petrofacies is intercalated within the shale facies of Um Abas Member at Wadi Khashir. The rock is yellow to yellowish-grey and moderately hard with organic burrows. It is dominated by quartz (60%) with small amounts of mica and plagioclase feldspars (4%) with few undefined bioclasts embedded in clay and calcareous matrix (45%, Fig. 4j). The grains are silty to medium sand size, subangular to subrounded, moderately to poorly sorted, open packing, and matrix supported.

The arkosic sandstone petrofacies is recorded in the upper beds of Um Abas Member, at Wadi Khashir and intercalated with shale/clay of the Siyatin Formation at Wadi Abu Ghusun and Ras Honkorab. It is light green, dark brown, or black colors, hard, and laminated with a thickness of about 4m which changes into evaporitic with black solid hydrocarbons in the Siyatin Formation at Wadi Abu Ghusun. The mineralogy of this petrofacies is composed of quartz (65%), mica (20%), feldspars (10%, Plagioclase, and microcline), and rock fragments (5%) embedded in calcite cement and argillaceous matrix (Fig. 4k). The Grains are fine to coarse, angular to subrounded, moderately sorted, and exhibit close packing, and straight-to-point contacts.

The sandstone facies of the Pliocene-Pleistocene age includes the greywacke and arenite petrofacies. The greywacke petrofacies is dominant and recorded as bedded sandstone in the Samh Member with a thickness ranging from 1m to 5.5m. It includes three petrofacies: feldspathic lithic and bioclastic feldspathic greywackes with quartz wacke. The feldspathic greywacke is semi-consolidated, bioturbated, burrowed, has a muddy nature, and is composed mainly of mono and polycrystalline quartz (50–70%), plagioclase and microcline feldspars (from 11 to 15%), and lithoclasts (from 2 to 35%) with mica flakes and bioclasts which are embedded in an argillaceous and calcareous matrix (Fig. 4l). The bioclastic feldspathic greywacke petrofacies is distinguished by algal fragments and carbonate intraclasts which are scattered between siliciclastic detrital grains in Gabir and Dashet El Dabaa members. Some algal fragments are completely dissolved forming moldic pores and quartz/feldspar grains have been fractured, partially to completely leached, and replaced by calcite cement (Fig. 4m).

The Arenite petrofacies is represented by the feldspathic lithic arenite petrofacies with 12m and 14m thickness in the Gabir and Samh members, respectively, with intercalations of pebbly conglomerates and siltstones beds. It is composed mainly of quartz (from 65 to 75%), microcline and plagioclase feldspars (from 18 to 25%), rock fragments (from 7 to 15%), mica flakes (biotite and muscovite), and small amounts of bioclasts embedded in argillaceous matrix (from 5 to 8%). The sandy grains are subangular to subrounded, close packing with point to straight contacts, moderate to well sorted, and range in size from fine to medium sand size (Fig. 4n). Tabular pebbles, micas, and silicified detrital wood grains are oriented parallel to lamination indicating a paleocurrent direction. At Wadi Mastoura, this microfacies is highly stained with black and yellow oil material inclusions between quartz grains (Fig. 4o).

The shale facies of the Miocene age is represented by the bioclastic and evaporite shale petrofacies. The bioclastic shale petrofacies is recorded in the western parts of Um Abas Member at Wadi Khashir, Wadi Abu Ghusun, and Ras Honkorab with a thickness of 4m, 2m, and 10.5m respectively. It is found as lamina of silt and clay about 1mm thick each and is composed of silty quartz grains, micas, detrital bioclasts of plant remains and micritized skeletal fragments of algae and foraminifera, and minor gypsum nodules (Fig. 5a). The evaporite shale petrofacies is interbedded with the Siyatin sandstone at Wadi Abu Ghusun and Ras Honkorab with a thickness of 6m and with Abu Dabbab evaporites at Wadi Mastoura with a thickness of 2.5m. It is made up of quartz, feldspars, mica, and rock fragments scattered within yellowish-green clays (Fig. 5b).

The siltstone petrofacies of the Pliocene-Pleistocene sediments is widely distributed within the sediments of Samh and Gabir members, at both Wadi Khashir and Wadi Mastoura with thicknesses ranging from 2m to 18m, respectively. It is variegated in color from brown, yellowish brown, light greenish-grey to red, semi-consolidated, and has a muddy nature and burrows that are filled with later calcareous cement (Fig. 5c). It is composed of coarse silty quartz grains, plagioclase, microcline, and mica flakes embedded in clay and calcareous matrix (Fig. 5d & e).

4.1.2. Carbonates microfacies

The Miocene carbonates microfacies are varied and include mudstone, wackestone, packstone, grainstone, and boundstone microfacies. These microfacies are strongly affected by the dolomitization process.

The mudstone microfacies is represented by brecciated dolomitic mudstone microfacies in the lower part of Um Mahara Formation at Wadi Mastoura with a thickness of 5m. The rock unit is hard, has a white creamy color, and contains fractures that may be formed by quite intense post-sedimentary tectonics. It is mainly composed of brecciated mudstone carbonates and algae fragments which are poorly sorted, mostly angular, and rounded with a grain size of 2cm (Fig. 5f).

The wackestone microfacies is represented by arenaceous (foraminiferal/molluscan) algal wackestone and recorded in the carbonates of Ranga Formation at Wadi Khashir and Um Mahara Formation in all measured sections. The rock unit is hard, bedded (< 1m to 3m in thickness), and yellowish brown to dark brown and characterized by algal lamination structures in Ranga Formation and highly fractured, mottled, and laminated in Um Mahara Formation. It is composed of siliciclastic grains, algal bioclasts (micritized and dolomitized), pelecypods, and foraminifera tests. All components are embedded in a micrite matrix that is recrystallized and dolomitized (Fig. 5g).

The packstone microfacies is represented by 1 to 3m thick arenaceous molluscan foraminiferal algal packstone microfacies in the Ranga and Siyatin formations at Wadi Abu Ghusun and Ras Honkorab. It is composed mainly of algal fragments, mollusks, foraminifera, and siliciclastic embedded in a little dolomitized micrite matrix (Fig. 5h). Parts of the molluscan fragments are recrystallized into a mosaic of equigranular sparry calcite crystals while the others are micritized and dolomitized (Fig. 5i).

The grainstone microfacies is represented by pelecypodal foraminiferal algal grainstone in the Um Mahara Formation at Wadi Abu Ghusun. It is composed of complete tests and bioclasts of pelecypods, foraminifera (operculina and nummulites), echinoid spines, and micritized algal fragments. All these components suffered dissolution forming moldic porosity, exhibit grain-supported fabric, and are embedded in dolomicrosparite and calcite cement (Fig. 5j & k).

The boundstone microfacies is the most abundant carbonates microfacies in the studied sections, and is represented by lithoclastic algal, algal coral, and molluscan algal boundstone microfacies. The lithoclastic algal boundstone is common in the Um Diheisi Member at Wadi Khashir and in the Um Abas Member at Ras Honkorab with a thickness ranging from 1 m to 9m. It has a dark chocolate brown color and a muddy nature, is nodular and laminated (Fig. 5l), and is dominantly composed of siliciclastic grains bounded by algal filaments of the lithophyllum type. Some siliciclastic grains are completely leached or partially replaced by calcite or anhydrite (Fig. 5m). The Algal coral boundstone microfacies form the reef core of Um Mahara carbonate facies at Wadi Mastoura, Wadi Abu Ghusun and Ras Honkorab areas with a thickness reaching up to 10m. The rock unit forms lenticular beds of yellowish-white limestone at the lower parts while it is dark grey to brown fractured, porous, and cavernous in the uppermost parts (Fig. 5n). Microstyolite structures are observed within beds of this microfacies at Wadi Mastoura and Wadi Abu Ghusun area (Fig. 5o). The algal stromatolite and coral heads are most common and increase upward. It is mainly composed of algal lamination and coral zoacia, where corals are strongly affected by aggrading recrystallization to blocky spar, and algae are affected by dolomitization (Fig. 6a & b). The Molluscan algal boundstone microfacies is recorded at the top of the Siyatin Formation at Wadi Abu Ghusun and Ras Honkorab, and Um Gheig Formation at Wadi Abu Ghusun with thicknesses reaching up to 5m. The rock unit is white to dark grey, hard, and porous. It is mainly made up of pelecypods and gastropod bioclasts that baffled within algae and corals (Fig. 6c). Upwards, this microfacies shows a diverse fossil assemblage of foraminifera, pelecypods, and coral bounded by algae. Algae and foraminifera are micritized while pelecypods are partially or completely leached to form mold pores that are filled with calcite or evaporite cement (Fig. 6d).

The carbonates microfacies of the Pliocene-Pleistocene sediments is represented by the mud-dominated and the boundstone microfacies. The mud-dominated microfacies is recorded as arenaceous bioclastic mudstone and wackestone. The arenaceous bioclastic mudstone microfacies is intercalated with the fine siliciclastic succession of the Samh Member, at Wadi Khashir. It exhibits a light brownish color, hard, porous, and massive nature with a thickness of 3.5m. This microfacies is made up of micrite with few bioclasts of molluscs, corals, and algae (Fig. 6e). The arenaceous bioclastic wackestone microfacies is recorded at the base of the Abu Shiqeili and Samh members at Wadi Khashir and Wadi Mastoura with a thickness of 2m. The rock unit is very hard, massive, and has a light green color with small brown and black patches. It is mainly composed of siliciclastic and intraclastic grains with bioclasts of molluscs, algae, and coral embedded in a micrite matrix (Fig. 6f). The micrite matrix and bioclasts were dolomitized. The anhedral sparry calcite and mottled dolomite crystals are filled and lined in the vuggy pores. The boundstone microfacies is represented by both the algal lithoclastic and coral boundstone. The lithoclastic algal boundstone microfacies is recorded at the eastern parts of the Dashet El Dabaa Member at Wadi Mastoura with a thickness of 4m. It has primary lamination and cross-lamination sedimentary structures. The rock unit is light yellowish in color and contains black solid hydrocarbon inclusions that were concentrated and filled the interspaces between grains (especially coarse pebble size) and parallel to lamination structures. It is highly porous with meso- to macro-pore size which may be enhanced by diagenetic effects and composed of micritized algae which surround and bound all the siliciclastic grains (Fig. 6g). The lithoclastic coral boundstone is represented by lenticular build ups (0.5m thick) that change westward to bedded coral reefs in the growth position in the Dashet El Dabaa Member at Wadi Mastoura. The coral beds increase in thickness westward (about 10m thick) with intercalations of medium to coarse siliciclastic sediments. This microfacies is composed of siliciclastic grains scattered and surrounded by coral that bound rock components together (Fig. 6h). The coral colonies are well preserved with the distinct structure of coral walls and septa.

4.1.3. Evaporite facies

The evaporite facies exists in the Miocene sediments only. It is easily distinguished by the obvious white color of the Abu Dabbab facies which is completely absent south of Wadi Mastoura. The evaporite facies cover large extent areas at Wadi Abu Ghusun and Ras Honkorab.

The Laminated anhydrite microfacies is dominant and well-represented within the Abu Dabbab Formation, at Wadi Mastoura and west of Ras Honkorab. At the base, it is mainly composed of intercalations of white glassy gypsum laminae and brownish-yellow clastics and calcareous materials (about 2-3m thick, Fig. 6i). The rock unit is composed of epigenetic felted masses of anhydrite laths that set in a mosaic of interlocking anhedral gypsum crystals indicating the secondary origin of the anhydrite (Fig. 6j). Clastic deposits consist of thin layers of silt, clay, or marl interlaminated with evaporites. Upwards, this microfacies grades into thin laminated yellowish-white anhydrite layers without any clastic laminae (stromatolitic anhydrite) that attains about 0.5m thick (Fig. 6k).

The Nodular evaporite microfacies is the most dominant and distributed microfacies of the Abu Dabbab Formation at Wadi Mastoura, Wadi Abu Ghusun and Ras Honkorab with thicknesses of 20m, 30m, and about 27m respectively, and white creamy color. The nodules are of variable size, but generally more than 1cm long. The evaporite nodules are vertically or randomly aligned and surrounded by a film of fine clastic materials or halite. It consists of discrete and elongated anhydrite nodules strung out along bedding planes to produce discontinuous, but clearly defined, anhydrite layers, separated by thin rims of fine clastic materials (Fig. 6l). The anhydrite nodules are composed of irregular felty epigenetic anhydrite crystals enclosing corroded and engulfed gypsum crystals indicating their replacement origin.

The Stellate evaporite microfacies is recorded in masses sporadically distributed within the Abu Dabbab evaporites of Wadi Mastoura. The anhydrite nodules show fibro-radiating or stellate structures with a diameter of about 35cm (Fig. 6m).

4.2. Petrophysical analysis

4.2.1. Porosity type

Both primary and secondary porosity are recorded in the studied sediments. The primary porosity is represented by interparticle/intergranular and intraparticle porosities. The interparticle porosity that reflects a primary origin is represented by intergranular pores in siliciclastic and grain-dominated carbonate microfacies and intercrystalline pores in mud-dominated and evaporite microfacies. The size of the pores is controlled by either the grain morphology and fabrics of grain-dominated carbonates and siliciclastic facies or the crystal size, textures of the evaporites, and micrite matrix of carbonates. It is common in the sandstones and conglomerate facies of Um Abas Member (Ranga Formation, Fig. 6n & o) and ranges from 3–20%. The intergranular pore spaces of polymictic conglomerate microfacies of Um Abas Member reach 5% as large interparticle pores are obstructed with dolomicrosparite to sparite crystals. In the feldspathic lithic arenite microfacies of Um Abas Member at Wadi Abu Ghusun the porosity increases by the dissolution effect on the dolomitized micrite matrix content (Fig. 7a). The interparticle porosity of the greywacke microfacies of Um Abas Member is low because of the high syn-depositional clay content and post-depositional dolomite and ferruginous cement. However, the porosity is enhanced during later dissolution diagenetic processes that leached dolomite between framework grains of the packstone microfacies (Fig. 7b). The intercrystalline porosity of the lithoclastic algal boundstone microfacies of Um Diheisi Member is recorded between the spheroidal dolomite crystals which are partially filled by ferruginous cement that closes the connection between pores (Fig. 7c). In the algal coral boundstone microfacies of Um Mahara Formation, the preservation of primary porosity is poor, to some extent, because of the destructive diagenetic cementation stages. The intercrystalline porosity is abundant between anhedral dolomite crystals in the wackestone microfacies of Um Mahara Formation (Fig. 7d). The dissolution processes of the feldspar grains enhanced the interparticle porosity in the feldspathic lithic arenite microfacies of the Siyain Formation (Fig. 7e).

The intercrystalline voids between gypsum crystals of the Abu Dabbab Formation are caused by recrystallization and differential internal shrinkage. In the Pliocene-Pleistocene sequence, the interparticle porosity is abundant in both greywacke and arenite sandstone petrofacies in the Samh, Gabir, and Dashet El Dabaa members. However, the interparticle pores of sandstones of Gabir Member are less preserved due to the greatly occluding by the presence of depositional argillaceous matrix and hydrocarbon inclusions (Fig. 7f).

The intraparticle porosity is abundant in the coral boundstone microfacies in the studied localities with distinct occurrence in the algal coral boundstone microfacies of Um Mahara and Siyatin formations at Wadi Mastoura (Fig. 7g). Recrystallization and cementation processes minimized this porosity in the coral limestones at Ras Honkorab. Gastropod shells were dissolved, allowing intraparticle pores to grow in the molluscan algal boundstone and packstone microfacies of the Siyain Formation (Fig. 7h).

Secondary porosity is common in both siliciclastic and carbonate rocks. Vuggy pores are varied in size and shape as mostly they are irregular in shape and range in size from millimeters to centimeters and rarely reach up to 0.5 m in diameter (Fig. 7i). When vugs are connected by enlarged interparticle pores, microfractures and their pore throats very highly permeable rocks are produced (Fig. 7j). Vuggy pores are enlarged by intensive dissolution of the different components either the skeletal parts (algae and pelecypods), or the siliciclastic grains (quartz and feldspar) (Fig. 7k). On the other hand, cementation is an extremely important diagenetic process where vuggy pores size has been reduced by ferruginous, calcite and/or evaporite cements (Fig. 7l). Oversized vuggy pores formed by non-selective dissolution of rock components (grains and matrix) are common in greywacke microfacies than in arenite ones (Fig. 7m &n). Touching vuggy pores are represented in the studied facies by cavernous and fracture pores in algal coral boundstone microfacies of Um Mahara Formation at all studied sections and Um Ghieg Formation at Wadi Abu Ghusun area (Fig. 7o).

In the algal boundstone microfacies of Um Diheisi Member and Um Mahara Formation at Ras Honkorab, the fenestral porosity is recorded with sizes up to 2 mm and concentration along the microchannels that may be originated due to algal lamination structure. In certain cases, the connections between fenestral pores are partially or completely filled by black iron cement which result in a decrease in permeability (Fig. 8a). Grain fracturing resulting from the tectonic activity produced the channel/fracture porosity which is found in the laminated greywackes of Ranga Formation and most sandstone microfacies of Marsa Alam Formation as micro-channels. This porosity is common in the siltstone and greywacke petrofacies of Um Abas and Samh members and sometimes the channels are filled by gypsum cement forming gypsum veins (Fig. 8b). In the carbonate microfacies, the channel/fracture porosity is less abundant and if exists it may indicate the influence of the pressure solution diagenesis. The channel pores are common in the conglomerates of the Abu Shiqeili Member at Wadi Mastoura and represented by fractured siliciclastic grains (Fig. 8c).

The above-mentioned porosity types are also represented as microporosity in the different petrofacies/microfacies. The abundance of the microporosity is variable depending upon the rock unit as it is high in the sandstone petrofacies and low in the evaporitic shale petrofacies of the Um Abas Member. In the conglomerate of the Ranga Formation, microporosity is low with the presence of the dolomite cement and vice versa. In the carbonate microfacies, the microporosity is high in the wackestone microfacies and low in the packstone microfacies as a function of the recrystallization process and the amount of the micritic matrix. Abu Dabbab evaporites have low microporosity values in the glassy gypsum, moderate values in the laminated evaporites, and maximum values in the nodular anhydrite.

The contribution of microporosity to the total porosity is controlled by the depositional texture, original mineralogy, and grain microstructure (Fig. 8d&e). The microporosity increases as the depositional texture has more mud content. Mineralogically, the microporosity is high in the carbonate microfacies rich in metastable phases such as aragonite and high-Mg calcite.

4.2.2. Pore-throat

Pores are connected by pore throats that vary in size and shape depending on their geological origin. The pore-throat size of the different studied lithofacies was determined from the cross-plotting of the relation between mercury injection porosity and air permeability. The conglomerates of Um Abas and Um Diheisi members (Ranga Formation) have the largest pore-throat size of up to 20µm (Fig. 9a&b). Pore-throat sizes of the greywacke range from 1.2µm up to 20µm owing to dissolution that enhanced the connection between pores and created vugs. Arkose and arenite have small pore throat sizes from 1.2µm to 1.6µm and from 2.8µm to about 6.3µm due to later occluding diagenetic cement that closed interconnection between pores. Boundstone of Um Diheisi Member has a pore-throat size from 5µm up to 20µm, and wackestone pore-throat size ranges from 0.48µm to about 20µm due to primary lamination and later fracturing which enhanced connectivity between pores. The packstone of Um Diheisi Member has a pore-throat size from 5µm to about 6µm.

The boundstone microfacies of the Um Mahara Formation has high pore-throat size ranging from 1.3 to 18µm. These values are affected by later diagenetic calcite, ferruginous and evaporite cement especially the ferruginous cement that filled and occluded the connections between different pores. The pore-throat size of the mudstone microfacies ranges from 0.9µm to 6µm, while it is very small in the wackestone microfacies of about 1.1µm. The grainstone microfacies has a pore-throat size of nearly 3µm (Fig. 9c).

The Boundstone microfacies of the Siyatin Formation has a pore-throat size of 0.5µm to 15µm. Arenites have a pore-throat size from 3.5µm up to 20µm, the pore-throat size of shales is about 2µm, whereas the pore-throat size of conglomerates is very low to negligible owing to evaporite cement (Fig. 9d). The evaporite facies of the Abu Dabbab Formation has various pore-throat size, that reaches up to 12µm in the glassy evaporite, about 1.8µm in the laminated evaporite, and range from 6µm to 15µm in the nodular evaporite (Fig. 9e). The pore-throat size of the boundstone microfacies of the Um Gheig Formation ranges from 3.5µm to about 16µm (Fig. 9f).

The siliciclastics of the Marsa Alam Formation (Samh and Gabir members) have various pore throat sizes. The conglomerates’ pore-throats are high and vary from 6.25µm up to 20µm (Fig. 10a). The pore-throat size of the greywacke microfacies varies from 0.7µm to 16µm (Fig. 10b). The arenites of the Gabir Member have pore-throat size from 0.75µm to about 16µm and are smaller in the arenites of the Samh Member and range from 1µm to 3.5µm. It ranges from 0.49µm to about 18µm in the siltstones of the Samh Member and decreases in the siltstones of the Gabir Member ranging from 0.8µm to 2.5µm. With respect to the Abu Shiqeili Member, the conglomerates’ pore throat size is high and varies from 1.9µm up to 20µm, the wackestone microfacies have a pore throat size of 20µm while the boundstone microfacies has a smaller pore throat size of 1.5µm (Fig. 10c). the Boundstone microfacies of the Dashet El Dabaa Member has a pore throat size ranging from 4µm to 16µm, whereas the greywacke microfacies has about 15µm pore throat size (Fig. 10d).

4.2.3. Measured porosity

The porosity values of the studied Ranga Formation range from 0 to 33.2% with an average of 13.7% (Table 1), whereas in the siliciclastic facies of the Um Abas Member, the values vary from 1.3 to 33.2% with an average of 14.5%. The highest values are recorded within the greywacke, feldspathic lithic arenite, and shale microfacies due to the presence of fractured grains and channel pores. On the other hand, the lowest values are recorded in the highly cemented quartz arenite, arkose, and conglomerate. The carbonates of the Um Diheisi Member have porosity ranges from 1.2 to 28.6% with a mean of 13.9%. The maximum values are recorded in the boundstone at Ras Honkorab due to the impact of the dissolution processes, while the packstone and wackestone microfacies have low porosity values.

Table 1

Petrophysical parameters of the studied formations.
	Ranga Fm.		Um Mahara Fm.		Siyatin Fm.		Abu Dabbab Fm.		Um Gheig Fm.
	Ø (%)	K (mD)	Ø (%)	K (mD)	Ø (%)	K (mD)	Ø (%)	K (mD)	Ø (%)	K (mD)
N	42	31	40	40	15	15	4	4	5	5
Min	0	0	1.5	0	3.3	0	7.2	10	1.3	0
Max	33.2	3806	46.1	1190	27	3320	38.8	962	17.3	514
Average	13.7	618	21.4	209	16.2	356	23.1	322	10.5	185
SD	8.8	1105	10.5	328	6.9	867	14.1	432	7.1	242
CV	0.64	1.79	0.49	1.57	0.42	2.44	0.61	1.34	0.68	1.31

In general, the Um Mahara Formation is characterized by porosity values ranging from 1.5–46.1% with an average of 21.4% (Table 1). At Wadi Mastoura, porosity values range from 6.93–44.2%, with an average of 22.2%. At Wadi Abu Ghusun, porosity values range from 10.5–46.1%, with an average of 22%. While, the porosity values of the Um Mahara Formation at Ras Honkorab area, range from 1.5–35.4%, with an average of 20.2% (Table 2). So, high porosity values of the Um Mahara Formation (boundstone microfacies) are recorded at Wadi Mastoura area than both Ras Honkorab and Wadi Abu Ghusun.

Table 2

Petrophysical parameters of the Um Mahara Formation at the studied areas.
	Um Mahara Formation
	Wadi Mastoura		Wadi Abu Ghusun		Ras Honkorab
	Ø (%)	K (mD)	Ø (%)	K (mD)	Ø (%)	K (mD)
N	13	13	11	11	16	16
Min	6.9	0	10.5	0	1.5	0
Max	44.2	1129	46.1	1190	35.4	791
Average	22.2	133	22	160	20.2	304
SD	12.8	308	9.6	357	9.6	321
CV	0.58	2.31	0.44	2.23	0.48	1.05

The porosity of the Siyatin Formation ranges from 3.3–27% with an average of 16.2%. These values depend on the amount of gypsum cement, where the minimum values are found in the conglomerate, arenite, and shale petrofacies which have a relatively high cement content. Abu Dabbab Formation displays values varying from 7.2–38.8% with an average of 23.1%. The maximum values recorded in the nodular anhydrite (38.8%) are due to their enrichment in the interparticle pores. The Um Gheig Formation has a porosity range from 1.3–17.3% with an average of 10.5% (Table 1). The highest values are recorded in the molluscan coral boundstone microfacies in the Wadi Abu Ghusun area.

The Measured porosity values of Samh, Gabir, and Abu Shiqeili members of the Marsa Alam Formation range from 0 to 25.7%, 2–27.9%, and 4.3–20.4% with average values of 14.5%, 15.4%, and 12.3%, respectively. In the Saghra Formation (Dashet El Dabaa Member), the porosity values range from 6.1–17.3% with an average of 11.4% (Table 3). In the Marsa Alam and Shagra formations, the porosity values are largely dependent on the facies type. It ranges from 2–27.1%, 7.9–24.1%, 4.2–25.7%, 4.3 to 27.9%, 4.9–11.3%, and 6.1–19% in siltstone, arenite, greywacke, conglomerate, mud-dominated and boundstone microfacies, respectively. The highest porosity values are found in siltstone and sandstones whereas the lowest is recorded in carbonates. The lowest porosity values might be attributed to the calcareous, solid hydrocarbon, and gypsum cements, which filled the interparticle, and vuggy pores. The porosity of the siliciclastics is reversely proportional to the matrix content. In conglomerates, the high porosity values are thought to be due to the coarse grain size (granule to pebble), the well-sorting, and the high textural maturity, while the lower porosity values are due to cementation. A dense dolomicrite of the mud-dominated rock unit and later dolosparite calcite and ferruginous cement are the main causes of low porosity. The good porosity of the boundstone microfacies (Shagra Formation) could be attributed to the intraparticle pores and their enhancement by dissolution.

Table 3

Petrophysical parameters of the Marsa Alam and Shagra formations’ members.
	Marsa Alam Formation						Shagra Formation
	Samh Mbr.		Gabir Mbr.		Abu Shiqeili Mbr.		Dashet El Dabaa Mbr.
	Ø (%)	K (mD)	Ø (%)	K (mD)	Ø (%)	K (mD)	Ø (%)	K (mD)
N	20	20	17	17	16	16	4	4
Min	0	0	2	0	4.3	0	6.1	36
Max	25.7	589	27.9	1466	20.4	3367	17.3	469
Average	14.9	73	15.4	239	12.3	681	11.4	257
SD	6.9	154	7.7	390	4.2	1047	4.9	235
CV	0.46	2.11	0.5	1.63	0.34	1.54	0.43	0.91

4.2.4. Measured permeability

The siliciclastic sediments of the Ranga Formation have permeabilities that vary from zero to 3806 mD with an average of 618 mD. The permeability of the shales of the Um Abas Member is zero in vertically oriented samples, while it ranges from 14 to 112 mD in the horizontally oriented samples that are parallel to laminations. The conglomerates have very high values that range from 550 to 3806 mD because they are semi-consolidated owing to the low cement content. The greywacke has higher permeability values (6 to 2516 mD) among the sandstones (0 to 66 mD) of the Um Abas Member because the greywacke is rich in microfractures, while the arenite and arkose are more cemented, and the pores are cut off and filled by calcite crystals. The permeability of the Um Diheisi carbonates ranges from 66 to 829 mD in boundstone, 0 to 190 mD in grain-dominated, and 1.4 to 360 mD in mud-dominated microfacies. The highest values are recorded in the samples which are rich in oversize and touch vugs. The Um Mahara Formation is characterized by permeability values ranging from 0 to 1190 mD with an average of 209 mD. These values range from 0 to 1190 mD in boundstone, from 0 to 356 mD in mud-dominated, and reach 24 mD in grain-dominated microfacies. The highest values are recorded in the coral boundstone due to the interconnection between the intraparticle and other pore types and in the fractured wackestone microfacies as well. The lowest values are recorded in the mudstone and grainstone microfacies due to their high cement content. The boundstone of Ras Honkorab area has high permeability values, while it is poor at Wadi Abu Ghusun. The permeability of the Siyatin Formation ranges from 0 to 3320 mD with an average of 356 mD. The maximum values are recorded in the coral boundstone microfacies at Wadi Mastoura and feldspathic lithic arenite at Wadi Abu Ghusun. In the Abu Dabbab Formation, it varies from 10 to 962 mD with an average of 322 mD. The highest values occur in the nodular anhydrite due to good interconnection between their interparticle voids. The Um Gheig Formation boundstones display values that vary from 0 to 514 mD with an average of 185 mD (Table 1). The maximum values occur in the samples, which have touch-vuggy pores. The permeability of the siltstones of the Marsa Alam Formation ranges from 0 to 4 mD in the vertically oriented samples and from 34 to 399 mD in the horizontally oriented samples (parallel to lamination). It ranges from 0 to 622 mD, 0 to 461 mD, 0 to 3367 mD, 0 to 623 mD, and 9 to 1903 mD in the arenite, greywacke, conglomerate, mud-dominated, and boundstone microfacies of the Marsa Alam and Shagra formations, respectively. The high permeability values were found in both conglomerate and carbonate facies and the lowest values were in the siltstones. Permeability also decreases in the Pliocene-Pleistocene facies owing to the closure of the pore-throat by bitumen and/or calcite filling. It increases in arenite despite their well-sorting and low to no matrix content (well maturity). While the permeability decreases in the feldspathic greywacke because their porosity is poorly interconnected due to the poor sorting and high matrix content. However, the non-selective leaching of matrix and grains increased the permeability of feldspathic arenite. The very high permeability values of some conglomerate microfacies are attributed to the interconnection of intergranular pores. These were influenced by each of the coarse grain sizes (granule to pebble), well-sorting, high textural maturity, and leaching of unstable silicate minerals. On the contrary, the poorly sorted and cemented conglomerates have lower permeability values. The high values of permeability are found in some sandstone and conglomerate samples due to the fracturing of the coarse silicate grains and bioturbation. The permeability of the coral boundstone microfacies is mainly dependent on the intraparticle pores which are affected by cementation, while the algal boundstone and mud-dominated samples are influenced by the recrystallization of the algal skeleton and calcareous matrix.

4.2.5. Porosity-permeability relationships

The presence of porosity doesn’t guarantee that permeability exists. The pores must be interconnected, and pore-throats must be large enough to permit the fluid flow. The pore spaces are reflected in measured porosity, while pore-throats are reflected in measured permeability. Thereupon, the geometric relationship between pore spaces and pore-throats defines the relationship between porosity and permeability. A semilogarithmic permeability versus porosity scatter plot of various lithofacies is constructed for the studied syn-rift formations. There is no strong correlation between the porosity and the permeability for all formations, as shown in Figure (11a-d).

Porosity-permeability heterogeneity is another aspect of a relationship. Saner and Sahin (1999), mentioned that the coefficient of variation (CV) which is the ratio of standard deviation to the mean is used as a heterogeneity indicator. A low CV represents a relatively homogenous population, whereas a high CV represents a heterogeneous population. A CV of less than 0.5 indicates homogeneity, between 0.5 and 1 reflects heterogeneity, and if the CV is greater than 1 it represents great heterogeneity (Corbet and Jensen, 1992).

From Tables (1–3), the CV values for porosity indicate that Um Mahara, Siyatin, Marsa Alam (Samh, Gabir and Abu Shiqeili members) and Shagra (Dashet El Dabaa Member) formations are homogenous, while Ranga, Abu Dabbab and Um Gheig formations are heterogeneous. Concerning permeability, the Shagra Formation (Dashet El Dabaa Member) is heterogeneous, while all other formations have considerably great heterogeneity. The porosity and permeability of Um Mahara Formation at Wadi Mastoura are heterogeneous. At Wadi Abu Ghusun and Ras Honkorab, the porosity is homogenous, while the permeability is heterogeneous (1.05) to highly heterogeneous (2.23).

5.1. The porosity and permeability as a function of the depositional texture

The porosity and permeability values of the conglomerate facies are higher (> 15% porosity and 100 mD) within the coarse-grained, well-sorted, and closed-packed samples of the Ranga and Marsa Alam formations. The lower values are recorded in the fine pebbly, poorly-sorted, and matrix-supported (open-packing) conglomerates. The porosity and permeability of the feldspathic lithic arenite and arkose of the Ranga Formation are affected by matrix content and sorting where the maximum values (33.2% and 82.4 mD) in the well-sorted and low to no matrix content of sandstones (especially arenite petrofacies).

The measured porosity and permeability of the studied carbonate microfacies of the Um Diheisi Member at Ras Honkorab are controlled by the depositional framework. The permeability is high in the coral-boundstone microfacies due to the primary (depositional) intrapore framework of the corals. The porosity of the grain-dominated microfacies is less than the mud-dominated microfacies because the latter is rich in matrix content (microporosity), while the permeability is larger in the grain-dominated microfacies due to the grain-supported texture and the little content of the primary pore-filling matrix.

The original framework of the boundstone microfacies of the Um Mahara and Siyatin formations has a great influence on the petrophysical characters. In the Um Mahara Formation, the maximum values are recorded in the boundstone, and the minimum values occur in the mud-dominated microfacies. However, the porosity and permeability values of the mud-and-grain-dominated microfacies at the Um Mahara Formation are affected by the diagenetic processes. The petrophysical properties of the sandstone facies of the Siyatin Formation are increased in the feldspathic arenite which is rich in interparticle pores indicating the effect of depositional texture. In some arenite and conglomerate samples of the Siyatin Formation, the porosity and permeability values reach minimum values due to cementation. The petrophysical properties of shale are controlled by the primary structure, where the highest values were noticed within the horizontally oriented measured samples.

The porosity and permeability values of the studied Marsa Alam and Shagra formations are found to be high in the sandstones whereas the lowest is recorded in the mud-dominated carbonates. The permeability is increased in the well-sorted arenite with low to no matrix content (well maturity). The permeability is decreased in the greywacke because the pores are poorly interconnected due to poor sorting and high matrix content.

The not-selective leaching of matrix and grains increased the permeability of feldspathic arenite. The high porosity and permeability values of some conglomerates are thought to have occurred due to the coarse grain size (granule to cobble), well sorting, well textural maturity, and the interconnection of intergranular pores. Moreover, the leaching of the unstable silicate minerals participated in enhancing the petrophysical properties (Critelli & Nilsen, 1996). In addition, the poorly-sorted and cemented conglomerates have low permeability values. The high values in some sandstone and conglomerate samples are attributed to the fracturing of the coarse silicate grains and bioturbation. Moreover, the original good porosity of the boundstone microfacies of the Marsa Alam and Shagra formations is represented by intraparticle pores and was improved by dissolution.

The relationship between the petrophysical properties (porosity and permeability) and the depositional texture is defined in the studied syn-rift siliciclastic and carbonates facies to evaluate the reservoir quality. The petrophysical characteristics of the siliciclastic facies are controlled by their composition and the depositional texture such as grain size, sorting, packing, and matrix/cement content. The depositional and petrological features of siliciclastic rocks such as the coarse grain size, high content of quartz and feldspars, and low clay content support the preservation of the primary intergranular pores (Qiao et al., 2020; Ali et al., 2022a; Civitelli et al., 2023). On the other hand, they depend on the fabrics of the studied carbonates, which are divided into mud-dominated, grain-dominated, and boundstone facies. The textural features and fabrics influence the amount, pore volume, pore-throat size, and interconnection of the primary porosity such as interparticle and intraparticle voids. These voids have a great effect on the permeability of the studied facies especially when they are not occluded by cement (Ali et al., 2018 & 2022b; Jiaqing et al., 2023).

5.2. The porosity and permeability as a function of the diagenetic processes

The rock fabric of syn-rift sediments in the investigated area was affected by the diagenetic processes i.e., dissolution, cementation, dolomitization, and recrystallization. The intensity of these processes might be related to the depositional texture and allochem types. The diagenetic processes might be influenced by the depositional textures of the coral boundstone and grain-dominated intensively than the algal boundstone and mud-dominated microfacies. In addition, the influence of these processes is higher in coarse siliciclastics than in fine siliciclastics.

5.2.1. Dissolution effect

The dissolution processes increased the porosity and pore-throat sizes of the syn-rift siliciclastic and carbonate facies. The dissolution and brecciation processes influenced the dolomitic mudstone microfacies of the Um Mahara Formation to form vugs and channel pores, which were the main causes of increased permeability. The dissolution of rock components along the tectonic fissures and fractures caused the formation of corrosion voids of varying sizes. The massive (non-selective) dissolution of rock grains and matrix in the sandstones of the Marsa Alam Formation played a very important role in creating vuggy pores size to oversized pores. These pores are connected by microchannels, which were developed due to tectonic effects and grain fracturing. The Bioclastic mudstone and wackestone microfacies of the Samh Member were affected by the partial to complete dissolution of the framework grains and matrix to form micro- and meso-sized pores. Selective and massive dissolution processes have a great enhancing role in creating secondary vugs, and oversized and channel pores in the sandstones and conglomerates of the Abu Shiqeili Member. The enlargement of interparticle pore spaces of the Abu Shiqeili conglomerates is another type of selective dissolution. The resulting enlargement of interparticle pore spaces improves flow characteristics and capillary properties. The dissolution process enhanced or increased the size of intraparticle pores in the lithoclastic coral-boundstone microfacies of the Abu Shiqeili Member. Dissolution (selective and massive types) participated in enhancing the intraparticle and creating vugs and moldic pores within the coral-boundstone microfacies of the Dashet El Dabaa Member. Thus, the reservoir characteristics of the studied sediments could be increased and enhanced with increasing intensity of the dissolution process.

The channel pores of the conglomerates of the Um Abas Member and the wackestone microfacies of the Um Diheisi Member are connections between fractured detrital grains and surrounding cement, which enhanced and increased permeability. The fracture and microchannel pores of the siliciclastic facies of the studied Pliocene sediments are developed through tectonic effect and grain fracturing during weathering and transportation processes. These pores may be developed later after ferruginous cementation. Fracture grains are connected to make a network system that extends to include the entire rock framework and increasing permeability.

According to Selley & Sonnenberg (2015), the dissolution process is very important in enhancing the reservoir characteristics of rocks. Generally, cementation reduces porosity and permeability, while the dissolution of cement and/or grains can reverse this trend. Pore enhancement by dissolution would create pore sizes and shapes with wide degrees of connectivity (Ahr, 2008). Dissolution might be selective or non-selective. Selective dissolution of aragonite shells, unstable silicate minerals (Critelli & Nilsen, 1996; Worden & Burley, 2003), and calcareous matrix will form moldic, and both separated and touch vuggy pores. In addition, the dissolution processes create the intraparticle and interparticle porosity, which are dominated, in the studied carbonate and siliciclastic facies. These pores are either connected by pore-throats or channels creating permeable rocks. Sometimes, partial dissolution and transportation of ions from particles to intercrystal pore spaces may have little effect on total porosity by reducing the intercrystal pore size and thus decreasing permeability.

The impact of the dissolution process on permeability depends upon the geometry and location of the resulting voids relative to the rock textures (Lucia, 2007). The dissolution of the framework grains may lead to an increase in porosity but seldom produces a significant increase in permeability (Siebert et al., 1984).

5.2.2. Dolomitization effect

The dolomitization of lime mud of mud-dominated microfacies probably increased their microporosity (up to 18.98%). Dolomitization may increase the particle size significantly, modifying the pore size distribution of the studied carbonates and smoothing out important petrophysical depositional textures. Some dolomitic bioclasts will be partially or completely leached to leave dolomite intercrystalline, moldic, and vuggy pores. These pore types increase the permeability values of the studied mud and grain-supported carbonates. Dolomitization is the second enhancing diagenetic process that creates intercrystalline pores between dolomite crystals of the Um Mahara Formation.

Replacement of limestone by dolomite can reduce porosity rather than enhance it (Lucia, 2000). The dolomitization effect recorded in the studied carbonates leads to an increase and/or reduction in the petrophysical parameters. The porosity and permeability increase with dolomite replacement, while they are decreased with progressive dolomite cementation (Ahr, 2008, Ali et al., 2018 & 2022b). In other instances, after the dolomitization of limestone, the supersaturated dolomitized solutions can continue to flow through the carbonate rocks, allowing precipitation of secondary dolomite cement, which reduces the porosity and permeability (Lucia & Major, 1994).

5.2.3. Recrystallization effect

The studied carbonates were affected by degrading and aggrading recrystallization processes. Micritized carbonate allochems (algae and molluscs) become richer in microporosity that was produced by the difference in the size of the original compared to the resultant micritized crystals. The aggrading process also enhances intraparticle porosity in the coral-rich boundstone microfacies. The carbonate mud recrystallization in the mud-dominated microfacies contributed to the increase of the intercrystalline pores. Sometimes, the recrystallized algae are represented by fractured algal grains that connect the pores. For example, fenestral pores of the algal boundstone of the Um Diheisi Member are connected by microchannels that may originate due to micritization of the algal lamination. Thus, the recrystallization process increases the porosity and permeability in the carbonate facies.

5.2.4. Cementation effect

The cementation process influenced all syn-rift sediments in the investigated area. The different porosity types are partially or completely reduced by calcite, dolomite, gypsum, salt, ferruginous, and hydrocarbon cement. The cement grows from pore walls into pore space and therefore reduces the pore size. The systematic pore size change and distribution affected the permeability values. These values are reduced by the occluding cement which is related to the abundance, crystal size, and spatial distribution of cement. Meniscus cement is confined to pore throats and reduces permeability more than would be expected from a similar volume of isopachous or equant cement. Thus, the cementation effect on the porosity and permeability of the studied facies depends on their crystalline geometry. Sometimes, the interparticle pores of the Ranga Formation are partially or completely filled by sparry carbonate crystals and ferruginous cement to decrease porosity and permeability. Coarse sparry calcite, iron oxides, gypsum, and salt are partially or completely filling vugs and channel pores of the Um Mahara Formation.

Moreover, the channel pores of the Siyatin Formation shale and sandstone are completely filled with evaporites causing a decrease in permeability. Some channel pores of the siltstones of the Samh Member are lined with ferromagnesian minerals that are squeezed from clay minerals and most of them are filled with evaporite. A dense dolomicrite in the mud-dominated microfacies and later dolosparite calcite and ferruginous cement of the Marsa Alam Formation are the main causes for decreasing the porosity. Porosity and permeability are also decreased in all Pliocene microfacies owing to the pore-throat closing by bitumen and/or calcite filling. Pore-filling anhydrite is typically pervasive and reduces both porosity and pore-size distribution of carbonate reservoir rocks because it occludes intergranular, intercrystal, and vuggy pore spaces (Lucia, 2007; Balaky et al., 2023; Civitelli et al., 2023). Poikiliotopic gypsum reduces porosity but does not reduce pore-throat size. Permeability is fundamentally controlled by the pore-throat size and remains near-constant, whereas the pore-filling aspect of patchy gypsum reduces porosity (Lucia, 2007).

5.3. Reservoir potentiality of the concerned syn-rift deposits

The porosity and permeability are important reservoir parameters, as being a function of reservoir fluid contents and production rates. These parameters of the studied sediments were controlled by the original depositional setting and resultant sediment fabric, diagenetic history, and the subsequent stratigraphy.

Based on the above petrophysical data, the quality of the syn-rift formations, if buried in their present state could be classified as fair to excellent reservoir rock characters (according to Leverson, 1967) whereas the Um Mahara and Marsa Alam (Gabir Member) formations were found to have high petrophysical quality. In order to give a full picture of the petrophysical quality of these different rocks, composite lithologic logs were constructed (Figs. 12–16). It is found that good permeable beds (storage units) are intercalated by impermeable ones which work as barriers or baffles. Thus, the studied syn-rift sediments might be a prospective reservoir for petroleum potentiality if it is buried. This study shows that significant porosities have been developed in the Middle Miocene carbonates (Um Mahara Fm.) and Pliocene (Gabir Mbr of Marsa Alam Fm.), and if deeply overburdened, might form good hydrocarbon reservoirs. This is in case coupled with favorable source and trapping conditions.

5.4. Basin evolution and regional correlation

The Red Sea rift basin was born on a Late Oligocene event (about 25 my; Allan and Charnock, 1965). in the southern Red Sea, the post-Eocene subsidence have been commenced during the late Oligocene and spread later to the northern Red Sea and Gulf of Suez in Egypt by the early Miocene (Hughes and Beydoun, 1992). The sediments of this time record the transition from continental to marine depositional environments, for the first time. The rifting activity was the major control of the Neogene sedimentation in the Red Sea area (Sallam and Ruban, 2020). The Red Sea syn-rift Abu Ghusun Formation is of Oligocene age and is composed of unfossiliferous conglomerates and conglomeratic sandstones, which is equivalent to the Nakheil Formation (mainly red siltstones and sandstones ) on the northern part of Egyptin Red Sea (Bosworth et al., 2020; El-Asmar and Abdel-Fattah, 2000). During the early Miocene (15–20 my), the active rifting was associated by accelerated subsidence (Bosworth et al., 2020), which was resulted in open marine deposition of the Ranga Formations. This event is known as mid-Clysmic tectonic event (16.5 my), during which the rift shoulders were uplifted (Richardson and Arthur, 1988). By the Middle Miocene (15 Ma), a marked decrease in rift kinematics may have led to the accumulation of extensive evaporates (Abu Dabbab Formation). These evaporites extended along the entire basin from Egypt to Eritrea and has been continued to until the end of the Miocene (5 Ma). Hughes and Varol (2021) attributed the precipitation of these evaporites, which was different from their counterparts in the Gulf of Aden (not evaporitic) to episodic isolation from the Indian Ocean during the Early/Middle Miocene (Planktonic foraminiferal zones N8-N9; Nannofossil zones NN4-NN5) transition. By the beginning of the Pliocene, seafloor spreading in the southern Red Sea enabled the intrusion of the Indian marine waters via the Bab el Mandab corridor, and thus, a new open marine conditions prevailed (boundstone microfacies of Marsa Alam and Shagra formations).

The pre-rift quartz-arenites are encountered in many areas on the Red Sea and Gulf of Aden Basins and were drilled in many wells in the Egypt part of the Red Sea. Mixed carbonates and silisiclastics are dominating the Syn- and post-rift successions on the Red Sea and Gulf of Aden Basins. However, hydroctabon trabs are not the same. The Neogene sediments include the hydrocarbon traps (i.e., post-evaporite sequence) on the Red Sea. In contrast, pre-rift Paleogene sequence the main play the Gulf of Aden and Neogene strata have only lower potential for hydrocarbons (Beydoun, 1989). This variation can be attributed to the tectonic configuration of different sub-basins that have controlled the sedimentation and the digenesis processes.

The study area extends between Hamata in the south to Ras Honkorab in the north with about 41 km in length and 10 km in width along the Egyptian Red Sea coast. The Miocene sequence (Ranga, Um Mahara, Siyatin, Abu Dabbab and Um Gheig formations) is composed of siliciclastic, carbonate, and evaporite facies. Siliciclastic facies are recorded in both Ranga and Siyatin formations with about 44.98% relative to all Miocene sediments. The Pliocene-Pleistocene (Marsa Alam, Shagra, and Samadai formations) sequence is composed of siliciclastic facies (89.4%) and subordinate carbonates (10.6%).
In the siliciclastic facies, conglomerate and greywacke display excellent petrophysical characteristics (Appendix 1) due to their depositional features including grain size and open packing in the conglomerate while in the greywacke the mineral composition, sorting, and matrix content are the governing factors. The digenetic dissolution process played a role in enhancing the porosity, pore throat, and permeability of these rock units.
The boundstone microfacies have the highest values of porosity and permeability among the carbonate microfacies (Appendix 1). This could be attributed to a combination of both depositional (texture) and diagenetic (dissolution) influences.
The depositional features charted the path for the subsequent diagenetic processes where the presence of feldspar in the composition of sandstone (e.g. feldspathic lithic arenite) paved the way for the dissolution process. Also, the depositional texture in the carbonate microfacies as in the coral-boundstone microfacies with intrapore porosity facilitated the effect of dissolution.
Hydrocarbon inclusions are recorded within the Upper Miocene-Pliocene sediments, which are represented by the Marsa Alam and Shagra formations, at Wadi Mastoura. Bitumen fills, partially or completely, the entire pore space in some samples.
The occurrence of surface bitumen in the Upper Miocene-Pliocene sediments, at Wadi Mastoura, sheds light on the hydrocarbon habitat and oil exploration for the Egyptian part of the Red Sea.

Competing interests

The authors have no competing interests to declare that are relevant to the content of this article.

Funding

This work was funded by the Researchers Supporting Project number (RSP2024R455), King Saud University, Riyadh, Saudi Arabia.

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Depositional and diagenetic drivers of a heterogeneous reservoir quality in the Neogene strata of the Red Sea (Egypt)

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Abstract

Figures

1. Introduction

2. Geological Settings

3. Materials and methods

4. Results

4.1. Facies analysis

4.1.1. Siliciclastic facies

4.1.2. Carbonates microfacies

4.1.3. Evaporite facies

4.2. Petrophysical analysis

4.2.1. Porosity type

4.2.2. Pore-throat

4.2.3. Measured porosity

4.2.4. Measured permeability

4.2.5. Porosity-permeability relationships

5. Discussion

5.1. The porosity and permeability as a function of the depositional texture

5.2. The porosity and permeability as a function of the diagenetic processes

5.2.1. Dissolution effect

5.2.2. Dolomitization effect

5.2.3. Recrystallization effect

5.2.4. Cementation effect

5.3. Reservoir potentiality of the concerned syn-rift deposits

5.4. Basin evolution and regional correlation

6. Conclusions

Declarations

References

Appendix 1

Additional Declarations

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